1. Field of the Invention
The present invention relates generally to a fuel supply control system for an internal combustion engine which is driven with a blended fuel composed of a plurality of mutually different fuels, such as gasoline and alcohol. More specifically, the invention relates to a fail detection of failure of a sensor for monitoring blending ratio of one of fuel, and fail-safe system for feedback control (.lambda.-control) of air/fuel ratio upon occurrence of failure of the blending ratio sensor.
2. Description of the Background Art
For anti-pollution purpose, various attempt has been made for using non-gasoline or gasoline containing mixture fuel for reducing amount of No.sub.x, CO and other pollutant. Typical blended fuel is gasoline/alcohol mixture. As will be appreciated, because of difference of octane value and combusibility of respective invidual fuels, fuel injection amount and spark ignition timing and so forth has to be adapted for the blending ratio of the fuels.
In case of pure gasoline, it has been considered that stoichiometric air/fuel is 15:1. On the other hand, stoichiometric air/fuel ratio in case of pure alcohol, e.g. methanol, is 5:1. Therefore, desired air/fuel ratio in the blended fuel is differentiated significantly depending upon blending ratio of the alcohol versus gasoline. Therefore, for precisely controlling fuel supply amount for the engine, it becomes essential to detect the alcohol ratio in the blended fuel so that stoichiometric or target air/fuel ratio can be precisely determined.
Various alcohol sensors have been proposed and put in the market for use in such purpose. Precision of monitored result of alcohol ratio in the blended fuel is important factor for determining precision level of the fule supply control, because of substantial variation of the desired or target air/fuel ratio for obtaining optimal engine performance. In other words, failure of the alcohol sensor in such fuel supply control system will become critical for such type of engine control.